I was asking myself that very question just the other day; thankfully the Slow Mo Guys have some sort of an answer for me in glorious, everything-is-better-in-slo-mo.
They don't actually say precisely how many, but it took them a full 20 minutes to put them all on, so I'm guessing quite a few. There's something quite beautiful about watching a watermelon explode at 2,800 frames a second. Literally everything is better in slo-mo. [YouTube via iO9]